Fix doxygen in cmake

[hannes-steffenhagen-diffblue]

This has been broken for 3 years, ever since doxygen.cfg was renamed to
doxyfile. This lets us build documentation in from cmake with the 'doc'
target. One difference to the previous version (i.e. the one that was
broken before) is that this creates the documentation in the cmake
binary directory now instead of the source directory. Reasoning for this
is that cmake targets should not change files in the source directory
ideally.

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Not addressed here: I think this is probably the wrong place to do doxygen stuff anyway, we're referring to files outside this directory which is somewhat unfortunate.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

@hannes-steffenhagen-diffblue Would you mind adding a test by invoking the target from one of the GitHub actions that use cmake?

[hannes-steffenhagen-diffblue]

@tautschnig how about that?

[codecov]

Codecov Report

Merging #5768 (b480661) into develop (0f57d77) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5768   +/-   ##
========================================
  Coverage    69.65%   69.65%           
========================================
  Files         1243     1243           
  Lines       100843   100843           
========================================
  Hits         70243    70243           
  Misses       30600    30600           

Flag	Coverage Δ
cproversmt2	43.39% <ø> (ø)
regression	66.56% <ø> (ø)
unit	32.20% <ø> (ø)

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[tautschnig]

Nit pick: ninja -C build doc will do the same in a single line.

[NlightNFotis]

Looks good to me